Known is an omnidirectional antenna mounted around a mast and having a reflector consisting of a number of panels, which are each provided with a radiator. The panels are grouped in a regular polygon around a mast, so that they fully enclose it. A drawback of this antenna is that the radiation diagram is not rotation-symmetrical. Problems particularly arise in the areas in which the radiation of one panel joins that of the next panel. In order to attain a desired field strength in those areas, the edge of the panel is irradiated at a lower level than its middle part. The consequence will be that part of the energy supplied by the radiator will be radiated beside the panel and will be lost.
According to another known construction for an omnidirectional antenna on a mast-head, the radiator has the shape of a round or cylindrical box, which is fed from the centre. The box wall inside the turned-up exit edge can be fixed in the middle to the tip of the reflector or to the enclosing box wall. However, when making use of such embodiments inadmissable distortions may easily occur. The box wall inside the radiator opening is preferably fixed to a material, e.g. synthetic material, that covers this radiator opening. The distorting influence of the synthetic ring on the radiation diagram of the radiator is negligible because the ring is narrow.
For an antenna mounted on a mast-head the radiator can be fed from the centre by means of a coaxial connection or a wave guide with a round cross-section. When, according to the invention, an antenna is to be placed around a mast, several methods of feeding the radiator can be thought of, but it is difficult to find an ideal construction.